[jon914]

Some farmed fish requires fish to feed them, which is unsustainable in itself when you're feeding N pounds of other fish to produce 1 pound of the edible type.

See: http://www.nicholas.duke.edu/thegreengrok/farmedfish-pnas092...

Recently, I've seen more of a shift to grains, but that reduces the nutritional value of the fish.

[netcan]

Why would shifting to grains reduce the nutritional value of the fish? Is cat more nutritious than lamb?

[jon914]

Fish feed consists of small, less desirable fish such as sardines and mackerel. These are rich in omega-3, so shifting to grains reduces the quantity available in the fish.

[netcan]

OK I get it. You didn't mean the nutritional value of a pound of fish.